It is known that the reception of acoustic telemetry signals travelling through the drilling fluid, often referred to as mud pulse telemetry becomes more difficult with increasing well depth, and with very viscous drilling fluids. Although some of the difficulty in signal reception is an inevitable consequence of the attenuation of the acoustic signal in its passage up the mud column, it is also impeded by the acoustic conditions at the top of the mud column inside the surface system.
The impeded acoustic conditions can take various forms, among these being reflections generated by equipment such as pulsation dampeners that reduce the received signal and noise from the mud pumps and other equipment that interferes with the signal.
Because of signal attenuation and impeded acoustic conditions in the surface system, the telemetry signal can often be degraded to a point where conventional mud pulse telemetry is either impossible or impractical.
U.S. Pat. No. 5,146,433 describes methods for recovering a LWD or MWD data signal in the presence of mud pump noise and generally comprises calibrating the mud pump pressure as a function of the mud pump piston position and then tracking the piston position during transmission of the LWD or MWD data signal and using the calibration information to subtract out the mud pump noise. However, a disadvantage of this type of method is that it requires a measurement of the mud pump piston position and fails if the pump noise changes after calibration.
U.S. Pat. No. 4,590,593 describes an electronic noise filtration system for use in improving the signal to noise ratio of acoustic data transmitted from a downhole transducer in a measurement while drilling system. It uses a delayed difference between two acoustic receivers to increase the signal to noise. A disadvantage of this type of system is that it does not adapt to changing acoustic conditions, and therefore the performance will ordinarily degrade over time.
U.S. Pat. No. 5,969,638 describes a system and method for signal processing of MWD signals. It uses multiple receivers with an optimised separation and with specified delays before combination to reduce the pump noise. However, a disadvantage of this type of arrangement is that all the receivers have similar signal to noise ratio.